The variable pitch blades of existing windmills and aeroplane propellers are carried in their hubs by ball or roller bearings so that they can be rotated or "feathered" by acceptable levels of torque applied either by manual or automatic means as required by considerations of application and design. These anti-friction bearings wear and pit, especially when subject to vibration while in one position for considerable periods of time. In present practice, the bearings of aeroplane propellers are inspected at regular intervals and replaced when worn. This maintainance is tolerated in an aeroplane, but would not be accepted in a windmill owned and maintained by average homeowners.
This invention discloses a torsionally flexible member which is strong and rigid in tension, within each blade. The outer end is fastened securely to the blade and the inner end is fastened to the hub to transmit the centrifugal force of the blade directly to the hub instead of thru the bearings providing nominal location of the blade, in the hub. In addition, the geometrical location of the blades in the hub is designed so that either output torque from a windmill, or torque to the blades of a propeller and aerodynamic thrust on the blades is transmitted directly to the hub as components of centrifugal force by the tension member within each blade.
When forces between the hub and each blade are thus transmitted wholly or in major part by a tension member instead of by the bearings locating the blades, sleeve bearings can be used with torque required to feather the blades within the output of a direct acting centrifugal govenor, or manual control. Sleeve bearings will provide long service without maintainance.
The tension member permitting use of sleeve bearings will also provide a valuable measure of safety. It will retain a blade accidentally separated from its root or hub, limiting damage to the immediate structure. This assurance of protection to persons and property near a windmill will increase public acceptance of the windmill as a means of putting the wind component of solar energy to work.
The tension member will be maintained ready to perform its safety function by constant use in feathering at excessive wind velocities. Any failure will be revealed by external inspection of the blade end or by malfunction of the overspeed governor. These factors insure that the tension member will be available when and if needed and it will be a reliable defense against damage to persons or property resulting from any unpredictable failure of the blade.
Transmission of torque is obtained by locating the centerline of blade force removed from the rotational centerline of the windmill or propellor so that torque from a windmill or to a propellor is transmitted as the product of the displacement radius and centrifugal force. By a similar geometry of mounting, each blade is located downwind at at "coned" angle to a shaft perpendicular, so that the aerodynamic thrust of the blade is also transmitted to the hub as a component of the centrifugal force of the blade.
When a tension member is provided to thus transmit all forces of a blade directly to the hub, sleeve journal bearings and thrust washers can be used to provide nominal location of the blade instead of the ball or roller bearings generally used when blade pitch is to be varied, while maintaining the low torque requirements necessary for adjustment of blade pitch. Sleeve bearings will furnish long life free of maintainance, in contrast to the periodic inspection and replacement of antifriction bearings which is tolerated in the propellers of aircraft, but which would be intolerable in a windmill owned and mintained by average-typical homeowners.
The same tension member provided to relieve the sleeve bearings of load, will be ready at any time to retain a blade which would otherwise be thrown from the windmill by accidental failure of attachment to the root or hub. Damage will be confined to the structure of the windmill to allay fear and increase public acceptance of windmills.